Sendmail relay config
To allow relay of certain host, add a new record in /etc/mail/access in sendmail server. For example, if you want your sendmail server to be used by an outsider who want to send email to another outsider(relay), add a record in /etc/mail/access file as follow: youcanrelaymyserver.com youcanalsorelaymyserver.net youcanrelaytoo.org